This invention relates to a tensioner adapted for giving tensile force to a toothed belt or a chain for driving a cam shaft of an engine, or relates to a tensioner adapted for giving tensile force to a belt or a chain for driving a sub-machine.
As shown in FIG. 9, prior tensioner 60 comprises a housing 62, a plunger 68 which divides the housing 62 into a low-pressure chamber 64 and a high-pressure chamber 66, and a spring 70 which urges the plunger 68 frontward (in a right direction as viewed in FIG. 9). The numeral 72 denotes a check valve. The plunger 68 contacts, for instance, an arm with an idler roller (not shown). In this kind of tensioner 60, the plunger 68 slowly moves backward (in a left direction as viewed in FIG. 9) when an oil flow from the high-pressure chamber 66 to the low-pressure chamber 64 is restricted. On the other hand, the plunger 68 quickly juts out when an oil flow from the low-pressure chamber 64 to the high-pressure chamber 66 is not substantially restricted.
The spring 70 used in the tensioner 60 is an equal pitch spring or a twisted coil spring. The spring has usually a linear characteristic. A tensioner should act to suppress a scattering of the initial tension of the toothed belt or the chain (herein after referred to as `belt, or the like`). Therefore, a spring having a small modulus is often used.
Using a spring having a linear characteristic will cause such problems as slipping of a belt, or the like, or making noise when an engine starts. The tensioner is usually placed on a looser side of the belt, or the like. A tension of the belt, or the like, at the looser side thereof while an engine is in operation sometimes becomes greater than that while an engine is at rest. Therefore, the plunger takes more backward position while an engine is at rest than while an engine is in operation. Under such condition, a required tension will not be given to the belt, or the like, when an engine starts, thereby causing slipping of the belt, or the like, or making a noise.
By using a spring having a small modulus, an amount of backward movement of the plunger will be large when an engine stops. An oil flow resistance is proportional to the jutting out speed of the plunger. Therefore, if an amount of the backward movement of the plunger is large, the belt, or the like, will slip until a required tension is given thereto.
The same problems of this kind will arise in a swing type tensioner as disclosed in Japanese Patent Application Laid-open Nos. Hei 2-66356 and 2-72252. A tensioner of this kind includes a cam swingingly connected to a fixed shaft. The cam is given a rotating force toward the direction at which a tension of a toothed belt or a chain increases.
Thus, it is an object of the present invention to provide a tensioner with a spring wherein tensile force is given to a toothed belt or a chain, and wherein slipping of the belt, or the like, is restricted by using a spring having a nonlinear characteristic.